1. Field of the Invention
The present invention relates to a camera having a function of automatically detecting a focusing condition and a function of automatically performing an exposure, and more particularly, to a camera having a system capable of automatically focusing by using a light which passes through the photographing lens (TTL system).
2. Description of the Related art
As well known, cameras commercially available are classified into a single-lens reflex camera and an non-single-lens reflex camera in terms of the construction of the viewfinder thereof. The latter includes a camera generally called a compact camera.
The single-lens reflex camera has features or advantages that an object image observed in the viewfinder coincides with an object to be photographed; a focusing condition is viewable in the viewfinder; and a zoom lens or a lens having long focal length can be exchangeably mounted thereon.
Recently, a single-lens reflex camera having a device which detects a focusing condition using a light which has passed through the photographing lens thereof is commercially available. This system is hereinafter referred to as TTL system. The TTL system is capable of accurately detecting a focusing condition. That is, the focus detecting device employing the TTL system detects, advantageously, a defocus amount in a predetermined focusing plane and is capable of detecting a focusing condition irrespective of an error in the mechanism of a camera or an error made by a temperature change.
Thus, the single-lens reflex camera currently available is advantageous in that it is capable of accurately obtaining the in-focus condition as a photographer desires, however, requires optical instruments such as a pentagonal prism and a mirror box. Thus, the camera is large-sized and heavy. Further, the movement of a quick return mirror causes the lens back of a photographing lens to be very long. Accordingly, the photographing lens is large-sized.
In the non-single-lens reflex camera currently available, a viewfinder optical system and an object distance measuring device are mounted in the camera body independently of a photographing optical system. This system is hereinafter referred to as non-TTL system. Further, the object distance measuring device adopts a zone focusing system. Although the non-single-lens reflex camera is inferior to the single-lens reflex camera in that it cannot take a photograph as faithfully as a photographer desires and cannot obtain a in-focus condition so accurately as the single-lens reflex camera does. However, the non-single-lens reflex camera is compact and is easy to handle. Further, since no quick return mirror is required, the lens back of the photographing lens can be considerably shortened. Therefore, the photographing lens is compact.
Recently, a zoom lens is increasingly mounted on an non-single-lens reflex camera. It is expected that a zoom lens having a high magnification and a long focal length is mounted on the non-single-lens reflex camera in the near future.
As described above, the object distance measuring device of the non-single-lens reflex camera adopts the non-TTL system and the zone focusing system. Therefore, if the photographing lens having a high magnification and a long focal length is mounted thereon, an in-focus condition cannot be accurately obtained.
That is, the non-TTL system does not detect an actual focusing condition. Therefore, if a mechanism for stopping a lens has an error, the error causes a picture to be out of focus. The mechanism of a camera is inevitably attended by such an error, which leads to an error in detecting a focusing condition. If the zone focusing system is employed, images of objects in the vicinity of an edge of the focus zone are not appropriately brought into focus. If a focal length of a photographing lens is long, a depth of field is very shallow. As such, the images of objects tend to be out of focus further.
In a camera having a variable focus lens in which a focusing lens is varied in its movement depending on the focal length thereof, a lens movement amount is determined as follows: A focal length region is divided into several areas. Then, the lens movement amount of the representative focal length in each of the divided areas is determined as a representative lens movement amount of each of the divided areas. Accordingly, a lens cannot be accurately moved in a focal length between the representative focal length of a certain area and the representative focal length of an adjacent area.
As described above, the known single-lens reflex and non-single-reflex cameras have both advantages and disadvantages.